Societies and Academies

Abstract

LONDON. Physical Society, November 12.—Dr. C. Chree, F.R.S., president, in the chair.—P. V. Bevan: The absorption spectrum of potassium vapour. The method of studying the absorption spectrum was that used first by Roscoe and Schuster, and of late years elaborated by Prof. R. W. Wood. That the optically dense vapour has small density makes it possible to heat the metal in a tube, and to have enough vapour to show strong absorption of light with little distillation to the colder parts of the tube. A tube with quartz plate ends can be used, and the absorption spectrum studied with a quartz spectrograph. The most evident feature of the spectrum obtained is the appearance of the lines of the principal series. None of the lines of the two associated series appear, but additional channelled space spectra unrepresented in the emission spectra. Fifteen new lines have been obtained in the principal series by this method. In the invisible region there appears a channelled space spectrum in the red. This shows definite edges of bands towards the violet end of the spectrum. The wave-lengths of the edges of these bands were measured. When the ratios of these wave-lengths to that of the first member of the principal series are found, the values are found to be in the same range as the corresponding ratios as deduced from Wood's measurements on sodium absorption. This the author regards as evidence of connection between the channelled space spectrum and the principal series of lines. The remarkable feature of this absorption spectrum is the difference in the properties of the principal series lines from those of other, lines in the emission spectrum. Some other lines are in emission spectra far stronger than the higher members of the principal series, yet do not appear at all in the absorption spectrum. This fact may indicate that the metallic vapour at the low temperature of these experiments is in a different molecular state from its state in a spark or flame, or that in these latter cases chemical action is going on. and. the emission spectrum is not a simple elementary spectrum.—J. S. Dow: Some further notes on the physiological—principles underlying the flicker photometer. The author suggests that something may be learned regarding the physiological phenomena governing the flicker photometer by observing whether it is subject to certain physiological effects, such as the “yellow spot” and “Purkinje” phenomena. Experiments show that the effects referred to do occur, but are apparently much less marked. The author sviggests an explanation based on the assumption that the rod-elements on the retina, in addition to the peculiarities attributed to these organs as regards the perception of light and colour, also differ from the “cones” in the fact that they seem to receive a luminous impression more slowly,. and retain it longer than these organs. This peculiarity is of little consequence in an ordinary photometer of the equality of brightness type, but may play a part in the flicker instrument; it seems to explain why certain effects should be perceived more clearly in one case than in the other. According to this theory, we may imagine the flicker effect to be due to two distinct portions, received by the agency of the rods and. cones respectively. Under certain conditions the speed of a flicker photometer mav be supposed to be suitable for the use of “cone-flicker” but too high for the “rod-flicker,” which becomes fused into a steadv luminous impression, and thus does not affect the readings of the instrument. The author regards his experiments as being assentially of a suggestive character, remiiring more detailed examination. One must be cautious in seeking to draw deductions from cases of colour-blindness, as many different varieties of this affliction exist.—Dr. Edridge Green: Colour-perception spectrometer. This consists of an ordinary spectrometer with a single prism, fitted with two wave-length drums, which work two shutters placed in the focal plane of the eyepiece. By means of the shutters any part of the spectrum can be viewed at will, and the wave-lengths of the edges of the patch under observation can be read off from the drums. Dr. Green described how the instrument is used for testing colour-blindness, and referred to the superiority of the method over those usually adopted.—H. G. Savidge: Tables of the ber and bei and ker and kei functions, with further formulæ for their computation.